In an prior art, there are proposed the above-described high frequency heating apparatus adopting a constitution of detecting an input current supplied by a commercial power source by a current transformer and controlling constant an output of an electromagnetic wave of the magnetron by carrying out a pulse width control such that the input current becomes a predetermined value (refer to, for example, Patent Reference 1) and adopting a constitution of detecting a secondary side current of a boost transformer of a high voltage circuit and controlling an input current constant (refer to, for example, Patent Reference 2).
Further, there is also proposed a high frequency heating apparatus adopting a constitution of detecting a secondary side current of a boost transformer of a high voltage circuit by a current transformer and stopping to operate an inverter power source when an abnormality is brought about at the high voltage circuit (refer to, for example, Patent Reference 3).
[Patent Reference 1] JP-A-8-96947
[Patent Reference 2] JP-A-8-227791
[Patent Reference 3] JP-A-5-121162
In all of the high frequency heating apparatus, the current constituting an object to be detected is detected by the current transformer.
An explanation will be given here of the high frequency heating apparatus proposed in Patent Reference 2.
FIG. 9 is a circuit diagram showing the constitution of the high frequency heating apparatus proposed in Patent Reference 2. The high frequency heating apparatus shown in the drawing is constituted by a unidirectional power source portion 1, an inverter portion 2, a high voltage rectifier circuit 3, a magnetron 4, a switching rate detecting portion 5, a secondary side current detecting portion 6, a control portion 7 and current transformers 8 and 9.
The unidirectional power source portion 1 is constituted by a diode bridge 101 for subjecting an alternating current power source from a commercial power source 20 into full-wave rectification, and a low pass filter circuit comprising a choke coil 102 and a capacitor 103. Further, in the unidirectional power source portion 1, the above-described current transformer 8 is interposed to a side of an alternating current input of the diode bridge 101 and used for detecting an input current. The inverter portion 2 is constituted by a resonant capacitor 201, a boost transformer 202, a transistor 203 and a commutating diode 204. The transistor 203 is operated to switch by a switching control signal of 20 through 50 kHz provided from the control portion 7. Thereby, a high frequency voltage is generated at a primary winding of the boost transformer 202. Further, the transistor 203 is referred to as IGBT (Insulated Gate Bipolar Transistor) by being formed integrally mainly with the commutating diode 204.
The high voltage rectifying circuit 3 is constituted by capacitors 301 and 302 and diodes 303 and 304 and generates a high direct current voltage by subjecting a voltage generated at a secondary winding of the boost transformer 202 to half-wave multiplying rectification to apply to the magnetron 4. The magnetron 4 is also applied with an alternating current voltage for a heater from a heater winding of the boost transformer 202. The magnetron 4 is brought into an emittable state by heating a cathode thereof by being applied with the alternating current voltage for the heater and generates electromagnetic energy when the high direct current voltage is applied thereto under the state. The high voltage rectifier circuit 3 is interposed with the current transformer 9 between a cathode of the diode 303 and the ground to be used for detecting a secondary current.
The switching rate detecting portion 5 detects an ON/OFF duty ratio of the transistor 203 of the inverter portion 3 and inputs a result thereof to the control portion 7. The secondary side current detecting portion 6 subjects the secondary current to full-wave rectification to detect an average value thereof and inputs a result thereof to the control portion 7. The control portion 7 multiplies an output signal of the switching rate detecting portion 5 by an output of the secondary side current detecting portion 6 to control the transistor of inverter portion 3 to ON/OFF such that a multiplied value becomes a desired value. In this way, the commercial power source 20 is converted into a unidirection voltage by the unidirectional power source portion 1, the converted voltage is converted into a high frequency voltage by the inverter 2 and boosted up by the boost transformer 202 and thereafter converted into a high direct current voltage again by being subjected to multiplying rectification by the high voltage rectifier circuit 3 to thereby drive the magnetron 4.
However, according to the high frequency heating apparatus of the prior art, the following problem is posed.
That is, the current transformer is used for detecting the input current, the current transformer per se is comparatively large-sized and therefore, the current transformer constitutes a hindrance in space saving formation, further, also the cost is comparatively high and therefore, the current transformer also constitutes a hindrance in a reduction in the cost.
Further, the current transformer is provided with a frequency characteristic and cannot detect a direct current in view of a structure thereof and therefore, when a position of inserting the current transformer is disposed at the alternating current input of the diode bridge 101 as shown by FIG. 9, a detection sensitivity differs by a difference between frequencies (50/60 Hz) of the commercial power source and therefore, when the input current is controlled by receiving an output of the current transformer at the control portion 7, reference signals need to provide in correspondence with the respective frequencies of the commercial power source.
Further, the current transformer is magnetically coupled with other magnetic circuit in view of the structure and therefore, the current transformer is liable to receive noise of the boost transformer 202 and there is a concern of inputting a signal including the noise to the control portion 7 to operate erroneously.
Further, the current transformer per se is constituted by a size to some degree and therefore, an interval of arranging the current transformer, the diode bridge 101 and the transistor 203 is prolonged to some degree and therefore, a wiring pattern on a printed board connecting these is also prolonged and noise can be generated. Also in this case, similar to the above-described, the control portion 7 is erroneously operated by the noise or an adverse influence is effected on a contiguous apparatus.
Further, although there is carried out a countermeasure against heat generation with regard to IGBT constituted by integrally forming the transistor 203 and the commutating diode 204 other than the diode bridge 101 on the printed board and the boost transformer 202 by using a cooling fan, a sufficient cooling efficiency cannot be achieved since the current transformers 8 and 9 having large shapes hamper flow of cooling wind and a cement resistor constitutes a heat generating source. Particularly, all the parts need to arrange in a limited space on the printed board in accordance with small-sized formation of a main body of the high frequency heating apparatus and therefore, an increase in the cooling efficiency is desired.